Write a migration guide.

Author: andriyDev

_release-content/migration-guides/load_builder.md

@@ -0,0 +1,21 @@
+---
+title: Advanced AssetServer load variants are now expose through a builder pattern.
+pull_requests: []
+---
+
+In previous versions of Bevy, there were many different ways to load an asset:
+
+- `AssetServer::load`
+- `AssetServer::load_acquire`
+- `AssetServer::load_untyped`
+- `AssetServer::load_acquire_override_with_settings`
+- etc.
+
+All these variants have been simplified to only two variants:
+
+1. `AssetServer::load()`: This is just a convenience and just calls the load builder internally.
+2. `AssetServer::load_builder()`: allows for constructing more complex loads like untyped loads,
+   loads including guards, loads with settings, etc.
+
+Every load variant above can be reimplemented using `load_builder`, and each one of these methods
+has deprecation messages on them explaining their new equivalent.